They require lots of skills, are competitive to get in, and they pay nothing at the beginning. Why?

I can partly understand that they may be trying to find out only those who are most motivated to join, but does that mean these careers are prohibited to people who might need a job for money, who might be in bad economic conditions?

I have more or less been educated for this field, but I am in serious economic trouble. I literally have no cash. Volunteer work would be suicide.